The present invention relates to mass transfer and heat exchange columns and, more particularly, to vapor-liquid contact trays used in such columns.
Various types of vapor-liquid contact trays have been developed to facilitate contact between ascending vapor streams and descending liquid streams in mass transfer and heat exchange columns having an upright, cylindrical shape. A plurality of such trays are normally placed in horizontally extending and vertically spaced apart relationship within an open internal region of the column. The trays contain a contacting area or active area having openings that permit vapor to pass upwardly through the tray deck for interaction with liquid flowing across the upper surface of the tray deck. The vapor-liquid interaction that occurs above the active area of the deck forms a froth that facilitates the desired mass transfer and/or heat exchange. Most types of trays also contain larger openings and associated structures referred to as downcomers that allow the liquid to be removed from the tray deck after interaction with the ascending vapor. The liquid is directed downwardly through the downcomer to a normally imperforate liquid receiving or inlet area on the underlying tray.
In vapor-liquid contact trays known as single-pass crossflow trays, the inlet area is typically located at one end of the tray and the liquid then flows across the tray to the opposite end of the tray where it enters a downcomer for passage to the underlying tray. In multiple-pass trays, two or more downcomers are provided on at least some of the trays and the liquid stream is divided into two or more streams that flow across any tray in opposite directions.
One problem with crossflow contact trays of the type described above is the difficulty in ensuring uniform flow of liquid as it travels across the tray from the inlet area to the downcomer. Because the trays are of a circular shape, liquid traveling along the perimeter of the tray travels a greater distance than liquid traveling along the center of the tray. As a result, areas of stagnant flow may develop and reduced liquid-vapor interaction and efficiency may occur.
Vapor-liquid contact trays disclosed in U.S. Pat. No. 3,045,989 to Kittel utilize a center downcomer and an outer annular downcomer on alternate trays. As a result of this downcomer arrangement, liquid flows radially or along a spiral flow path from the center to the perimeter of alternating trays and then from the perimeter to the center of the remaining trays, thereby providing a more uniform flow of liquid and increased efficiency and capacity. While the tray depicted in the Kittel patent represents a significant advance over other types of vapor-liquid contact trays, additional improvements are desirable.